Methods and apparatus for extracting metals from mined ore are generally well known. Metallurgical processes have been developed which, through a series of concentration steps, produce substantially pure metal suitable for use in final applications. For instance, copper ore typically contains minerals comprised of copper, sulfur, iron and oxygen, with the total content of copper rarely exceeding 5%. Through a series of metallurgical processes, high purity copper (99.997% and higher) is produced. The final process employed in this series is eletrorefining, in which a relatively impure copper anode is dissolved into an aqueous electrolyte through the application of electrical current. The dissolved copper is then deposited onto another surface to form high purity copper cathode. The tank in which this occurs is commonly referred to as an electrorefining cell.
Mature electrorefining techniques have emerged to meet the demand for large volumes of highly pure metals, particularly copper. In a typical electrorefining cell, a plurality of "impure" (e.g. 99.6%) copper anodes are interleaved among respective cathode plates upon which high purity copper is deposited. The impurities in the anode typically include, inter alia, gold, silver, selenium, tellurium, lead, bismuth, nickel, arsenic as well as mold release agents used to facilitate the removal of anodes from molds at the conclusion of the casting process.
An aqueous electrolyte fills and flows through the cell while a voltage differential is applied to the anodes vis-a-vis the cathodes. Typical aqueous electrolytes contain plating reagents to ensure a flat smooth cathode deposit, an important measure of cathode quality. In the process, insoluble anode constituents form a layer on the anode face; as refining progresses, some of this material then falls off and generally sinks to the bottom of the refining cell. Soluble species dissolved from the anode either stay in solution in the aqueous electrolyte or form precipitates which adhere to the layer on the anode face, or sink to the bottom of the cell. The solids, comprised of insoluble anode constituents and precipitated compounds are commonly referred to as "slimes" and are typically collected as a slurry in the bottom of the cells.
By carefully controlling the various process parameters associated with the electrorefining process, extremely pure metal cathodes may be obtained. The cost associated with constructing and operating large electrorefining facilities are, however, substantial. Hence, it is desirable to maximize the rate of production of high quality cathodes from an electrorefining facility.
The rate at which copper is dissolved from anodes and replated at the cathodes is directly proportional to the amount of electrical current applied to the cathodes and anodes in the electrorefining cell. The intensity of the applied current is commonly expressed as current density, typically having units of amperes per square meter. Hence, the cathode production rate from any given electrorefining cell may be increased by increasing the current density. However, there are practical limitations to increasing current density; lower quality cathodes are produced if the current density is increased beyond the capabilities of the technology employed.
The quality of the cathode is a function of, inter alia, the concentration of reagents in the electrolyte filling the volume between each anode and cathode. More particularly, it is desirable to ensure a substantially uniform reagent concentration throughout the entire electrolyte volume surrounding the anodes and cathodes within an electrorefining cell. The formation of a dense, smooth and flat cathode deposit is required to maintain the quality of the cathode and the efficiency of the process. Efficiency is lost when an irregular deposit is formed that causes the anode and cathode to make physical contact. When this occurs, current flows through the point of contact rather than causing the dissolution of anode and deposition of cathode. The energy consumed by this short circuit is wasted as heat in the electrolyte.
The temperature of the electrolyte within the cell also tends to influence the quality of the finished cathodes. Electrolyte is typically heated to 57.degree.-68.degree. C. to improve, inter alia, the conductivity of the electrolyte, the rate at which reactions occur in the cell and the viscosity of the electrolyte. Operating at increased temperatures generally has a salutary effect on the quality of the cathode produced and can also reduce the unit cost of production. Ideally, the temperature of the electrolyte would be uniform throughout the electrorefining cell, however, common electrolyte flow rates and delivery methods are inadequate and the temperature of the electrolyte can be several degrees different from one location to another within the cell. The consumption of reagents is also related to the temperature of the electrolyte; some of the reagents used tend to degrade and become less effective more rapidly at higher temperatures. Rapid degradation coupled with non-uniform distribution of electrolyte tends to result in lower quality cathode.
The purity of cathode is also a function of the amount of slimes occluded in the cathode during refining. Slimes occlusion occurs when particles of slimes that have broken off from the layer surrounding the dissolving anode become suspended in the electrolyte and migrate to the surface of the cathode. Copper is plated around and over the particle, thereby effectively incorporating the impurities comprising the particle into the mass of the cathode. Preferably these impurities sink to the bottom of the cell, thereby removing them from the active plating region and eliminating the possibility of them becoming incorporated into the cathode deposit.
The profitability of an electrorefining facility is inter alia, a function of the production rate of the facility, i.e., the rate at which pure cathodes are produced. As stated above, the rate of deposition of cathode is essentially a linear function of the amount of current applied to the anodes and cathodes. However, in order to ensure high cathode quality, substantially uniform reagent distribution and substantially uniform temperature must be maintained within the cell. Both of these parameters require a sufficient flow rate of electrolyte through the system to ensure adequate and uniform supply of plating reagents to the entire active area of each cathode, while reducing the residence time of the electrolyte within the cell and, hence, reducing the temperature drop of the electrolyte while resident in the cell.
Accordingly, it can be said that the intensity of the current density which may be properly applied to the electrodes depends on the ability of the system to provide a sufficient electrolyte flow rate and uniform reagent and temperature distribution throughout the cell to maintain high quality cathode production. However, the electrolyte flow rate may typically not be increased to the point where the slimes are disturbed; if the slime at the bottom of the cell or on the anode face is disrupted, the impurities which comprise the slime may be plated onto the cathode, dramatically compromising cathode quality.
A flow rate on the order of 5 to 10 gallons per minute (GPM) has evolved as the standard in the electrorefining industry. This flow rate is generally viewed as providing acceptable reaction times and adequate reagent delivery, while not unnecessarily disturbing the slime. In this regard, it is noted that flow rates in known electrowinning processes often approach 50 to 60 GPM, inasmuch as electrowinning processes typically do not involve the formation and accumulation of slimes; hence, turbulent, high velocity aqueous flow in electrowinning systems does not produce the same quality problems typically encountered in an electrorefining context.
Presently known electrorefining systems typically involve an electrolyte inlet port disposed at one end of the refining cell and an electrolyte discharge port disposed at the opposite end of the refining cell. These ports are typically configured as orifices of circular cross-section, of sufficiently large diameter to permit gravity pumping of the solution through the cell along a flow path generally perpendicular to the planes of the electrodes. By maintaining flows in the range of 5 to 10 GPM, the slime is kept from suspending in the electrolyte, resulting in substantially pure cathodes. However, inasmuch as the magnitude of the current density which drives the reaction is limited by the electrolyte flow rate, aggregate cathode production remains limited by the rate at which electrolyte may be uniformly pumped through the system.
A technique for enhancing the production of highly pure cathodes is thus needed which overcomes the shortcomings of the prior art.